The present invention relates to a commutation type DC breaker having a commutation circuit electrically connected in parallel to a main contact, and more particularly to one which inserts a back current by using a commutation capacitor chargeable directly from a DC main line.
A prior art back current insertion type DC breaker is described in "DC Breaker", Electricity Review, September 1980, pp 791.about.795.
Another prior art commutation type DC breaker is disclosed in JP-A-54-149873 and a circuit diagram thereof is shown in FIG. 1, in which a main contact 1 is connected to a DC main line 15 between a DC power supply 11 and a load 12, and a commutation circuit comprising a series circuit of a commutation capacitor 2, a reactor 3 and a commutation switch 4, and an energy absorption device 5 are electrically connected in parallel to the main contact 1.
A current detector 14 for detecting an abnormal current, when such an abnormal current flows through the DC main line 15, and an overcurrent tripping device 13 for commanding actuation of a control device 6 to open the main contact 1 are provided between an electrical connection point P, between of the main contact 1 and the commutation circuit, and load 12.
The break of the main circuit current is first detected by the current detector 14 which detects the abnormal current, and the control device 6 is actuated by the command from the overcurrent tripping device 13. Then, the main contact 1 is opened by the control device 6, and then the commutation switch 4 of the commutation device is closed to direct an oscillating commutation current, which is of opposite direction to that of the main circuit current, to the main contact 1 to form a zero current point with the main circuit current so that the current is broken in a manner similar to that of AC current break.
However, through the evaluation of the prior art commutation type DC breaker by the inventors of the present invention, it has been revealed that there is a bad affect to the load 12 in a non-load open mode. Namely, it has been found that in the non-load open mode, the main contact 1 recovers its pole-to-pole insulation substantially simultaneously with the opening of the main contact 1, and when the commutation switch 4 is thereafter closed, a commutation current I from the commutation circuit does not flow through the main contact 1 but flows into the load 12. As a result, if an operator checks the line of the load 12 assuming that the current break is over, there is a risk that the operator will an electric shock.